Twenty-four hour clocks are in current use, particularly in the aviation and military fields. Such clocks have a circular dial or face which is divided into arc points spaced by 15.degree. from each other around the dial to provide a total of twenty-four points, each representing an hour of the day. Generally, the top location, corresponding to the hour "12" on a twelve-hour clock, is designated as "0" or "24" on a twenty-four hour clock dial, with the numbers "1" through "23" progressing clockwise.
Such clocks have an hour hand, a minute hand and a second hand, which make one complete revolution each twenty-four hours, each 60 minutes and each 60 seconds, respectively. The hour hand may comprise a geographic radiant or disk which may include a local hour hand which is adjustable relative thereto to indicate the local hour time and the concurrent hour time in a plurality of continents illustrating geographic areas of the World. The minutes and seconds indicated by the other hands are universal.
Realizing the relationship between the rotation of the hour hand of a twenty-four hour clock and the rotation of the Earth relative to the Sun, it has been proposed to provide a twenty-four hour clock having a geographic disk or radiant, corresponding to the Southern Hemisphere, which rotates with the hour hand and thus provides a visual indication of the changing location of any or all of the continents of the Earth relative to the axis of rotation of the Earth and to the fixed position of the Sun, and a visual indication of the local time in each of said geograhical areas. If a Northern Hemisphere radiant was illustrated, the clock would be required to rotate in the unconventional counterclockwise direction. Hence, the Southern Hemisphere is illustrated, to coincide with the true clockwise rotation of the Earth relative to the Sun.
Reference is made to U.S. Pat. No. 694,256 which discloses a twenty-four hour clock which provides the viewer with a visual indication of the local time of day, shown by the position of the hour hand, and with a simultaneous visual indication of the concurrent time of day in each of the geographic areas shown on the geographic radiant or disk, indicated by the position of said geographic areas, relative to the location of the Sun which is pictured in fixed position on the dial face at the 1200 hour position (noon or prime meridian). The other twenty three hour positions, or alternate even-hour positions, are also indicated around the outer periphery of the dial face, each hour position being spaced from the next by 15.degree. increments. This conforms the clock face to a hemisphere of the Earth which, by international agreement has been divided into twenty-four time zones of 15.degree. longitude each. Greenwich, England has been designated time zone zero and is the Prime Meridian. Therefore, the Earth latitude which is positioned 180.degree. latitude from Greenwich, i.e., the Fiji Islands, New Zealand and the Bering Straights, is designated as the International Date Line. At Prime Meridian Greenwich is directly under the Sun, or at noon. This is the only instant of each twenty-four hour period in which the entire Earth is experiencing the same day or calendar date. However, the next second brings a new day or calendar date in the International Date Line time zone so that different portions of the Earth are simultaneously experiencing two different days or calendar dates.
Conventional twenty-four hour clocks are useful in the aviation and military fields for providing a universal reference for time, i.e., Greenwich Mean Time. Also, such clocks are very desirable for people who travel around the World frequently and/or who are in frequent contact with people in various other parts of the World, such as business people with business associates or companies located around the World. Thus, it is useful or necessary to be aware of the time of day in other parts of the World before attempting to make an international telephone call or before making airplane reservations or other commitments which can be more effectively carried out if planned to coincide with certain local hours and dates in other parts of the World.
However, conventional twenty-four hour clocks have at least one important limitation with respect to the visual display of World-wide time. As mentioned supra, a twenty-four hour clock encompasses two different calendar days at all times except at the instant when noon occurs at the Prime Meridian, i.e., in the Greenwich time zone, and midnight occurs concurrently at the International Date Line, time zone 12, 180.degree. latitude from Greenwich. Thus, while a twenty-four hour clock provides a quick visual indication of the time of day in other continents on Earth, it does not include any means for providing a quick visual indication of those portions of the Earth experiencing the same day as the local day and those portions of the Earth experiencing a different day. Such information is important in connection with the making of business telephone calls on a Friday or a Monday to parts of the World where it may not be a business day, i.e., Saturday or Sunday. Such information is also important in connection with meeting date deadlines, expiration dates and other commitments where the first or last days of a month or year may be critical.